6.1 Hubble's discovery and the expansion of the universe

Hubble's Discovery and the Expanding Universe

In the late 1920s, Edwin Hubble made observations that fundamentally changed how we understand the cosmos. By measuring the distances and motions of galaxies, he discovered that the universe is expanding uniformly in all directions. This single finding overturned the idea of a static universe and became the foundation for the Big Bang theory.

Hubble's Expanding Universe Observations

Hubble worked with the 100-inch Hooker telescope at Mount Wilson Observatory, which was the most powerful telescope of its time. His approach combined two key measurements: galaxy distances and galaxy redshifts.

Measuring distances with Cepheid variables: Hubble used Cepheid variable stars as "standard candles" to determine how far away galaxies were. Cepheids are stars that pulsate in brightness on a regular cycle, and there's a well-defined relationship between a Cepheid's pulsation period and its true luminosity. By comparing the true luminosity to the observed brightness, Hubble could calculate the distance to the galaxy containing that star.

Measuring redshifts from galaxy spectra: Hubble also analyzed the light spectra of galaxies and found that their spectral lines were shifted toward longer (redder) wavelengths. This is called redshift ($z > 0$), and it occurs because of the Doppler effect: when a light source moves away from you, the wavelengths of its light get stretched. A greater redshift means the galaxy is receding faster.

The key discovery: When Hubble plotted recession velocity against distance, he found a striking linear relationship. Galaxies that were farther away were moving away faster, in direct proportion. This is expressed as Hubble's Law:

$v = H_0 \times d$

• $v$ = recession velocity of the galaxy (km/s)
• $d$ = distance to the galaxy (Mpc, or megaparsecs)
• $H_0$ = the Hubble constant, which quantifies the expansion rate of the universe (km/s/Mpc)

This relationship held true in every direction Hubble looked, providing the first direct evidence that the universe is expanding uniformly.

Significance of Hubble's Cosmic Discoveries

Before Hubble, most scientists assumed the universe was static and eternal. Even Einstein had added a "cosmological constant" to his equations of general relativity to keep the universe from expanding or contracting. Hubble's observations changed all of that.

• Overturned the static universe model. The data showed the universe is dynamic, not fixed in place. (The steady-state theory, which proposed the universe looks the same at all times, was a later alternative that also lost support as evidence for expansion accumulated.)
• Implied a finite age. If the universe is expanding now, you can run the clock backward to a time when everything was compressed together. Current measurements place the age of the universe at approximately 13.8 billion years.
• Laid the groundwork for the Big Bang theory. The expansion pointed to an origin from an extremely hot, dense state, which we now call the Big Bang.
• Revealed the true scale of the universe. Hubble showed that galaxies like those in the Coma Cluster and Ursa Major Cluster lie far beyond the Milky Way. The universe turned out to be vastly larger than anyone had imagined.

Implications of Universal Expansion

The discovery of expansion raised a natural question: what happens next? The answer depends on how much matter and energy the universe contains relative to its expansion rate.

There are three possible fates, determined by comparing the actual density of the universe to a critical density:

1. Open universe — If the density is below the critical value, gravity is too weak to stop the expansion. The universe expands forever.
2. Closed universe — If the density exceeds the critical value, gravity eventually halts the expansion and pulls everything back together in a "Big Crunch."
3. Flat universe — If the density exactly equals the critical value, expansion slows down over time but never fully stops.

Current observations suggest the universe is very close to flat, but with a twist: the expansion is actually accelerating.

The standard cosmological model (Lambda-CDM) accounts for this by incorporating two additional components:

• Dark matter — An invisible form of matter that doesn't emit or absorb light but exerts gravitational pull. It explains why galaxies rotate faster than visible matter alone would predict, and it provides the gravitational scaffolding for large-scale cosmic structure.
• Dark energy — A mysterious form of energy that permeates space and drives the accelerating expansion. It makes up roughly 68% of the total energy content of the universe, yet its fundamental nature remains one of the biggest open questions in physics.